The present invention relates to fiber-optic sensors and particularly to distributed fiber-optic sensor arrays wherein multiple sensors are individually monitored.
Over the past few years, fiber-optic devices have been actively studied and developed for use in various sensing applications in a wide range of fields. One reason for this interest is the sensitivity of optical fibers to environmental conditions which surround them. For example, factors such as temperature, pressure, and acoustical waves directly affect the light transmitting characteristics of optical fiber. These changes in the optical fiber produce a change in the phase of light signals traveling in the fiber. Thus, a measurement of the change in phase of optical signals which have been transmitted through that fiber is representative of changes in those environmental conditions which have affected the fiber.
Recently, particular efforts have been directed to the development of systems having sensors organized in arrays, so that a number of sensors can utilize light from a single source, and provide environmental information at a common detection location. Ideally, such an array would consist of a fiber input bus which would distribute light comprising an optical carrier to a set of sensors. Each sensor would imprint information about the environment to this optical carrier. An output fiber bus would then collect this light from the sensors and carry it back to a central information processing location where information obtained from any selected one of the sensors could be readily identified and analyzed.
The goal of these development efforts is to produce sensor arrays which could be used for specific applications such as monitoring rapidly changing environmental conditions. For example, such sensor arrays could be used to detect acoustic waves in applications such as geophysical surveying, in order to determine the source location and acoustical characteristics of those waves. For many such applications, it may be necessary to space the arrays over a relatively large area. In these situations, the replacement of electrical lines by fiber optics, for example, would overcome problems such as electrical pickup, cable weight, and safety hazards associated with the use of those electrical lines. Even when the sensor is used in limited space, the removal of electronics and bulk optics components generally should provide improved system performance due to reduced noise.
Combining the returns from different sensors onto a single fiber minimizes the number of fibers used in the sensor array, thereby further reducing the bulk and weight of the system. However, this feature creates another design challenge by necessitating some technique by which information from each sensor can be separated for individual identification from among all of the information arriving at the central processing location on the single fiber. Distributed sensing systems developed previously have generally applied one of two approaches for separating information of an individual sensor from a single data stream.
The first approach comprises time-division multiplexing of the sensor outputs, as is described by A. R. Nelson and D. H. McMahon, "Passive Multiplexing Techniques for Fiber-Optic Sensor Systems," I.F.O.C., p. 27, March 1981. In time-division multiplexing, the optical input most generally is pulsed so that the input signal comprises a pulse wave form. As a result, each sensor produces a pulse which, as a consequence of the system geometry, is separated in time from the other sensor signals. Specifically, the optical input pulse communicated through each sensor is placed on the output fiber by each of the sensors at a different time. By controlling the relative position of the sensors, interleaving of the pulse signals may be accomplished as the signals are multiplexed from the sensors onto a return fiber bus. These interleaved pulse signals are then carried back to the central processing location where demultiplexing and further signal processing occur.
One problem which is inherent with this type of system is that the frequency at which the sensors may be monitored becomes limited by the number of sensors. Specifically, it is noted that a second pulse may not be transmitted from the optical source until a certain amount of time has passed. If the second pulse were transmitted through the sensor nearest the source before the optical signals from all sensors has passed the output terminal of that sensor, it is possible that signals resulting from the second pulse could pass through the first sensors in the array and be placed on the return bus prior to the passing of optical signals produced from sensors near the end of the array. This would, of course, prevent the demultiplexing and signal processing equipment from determining the relationship between the pulse signal received and its associated sensor. Such systems are, therefore, often not useful in applications requiring rapid repeated sensing of environmental conditions by each of the senors in the array.
The second approach which has been used for separating each sensor's information from the single data stream has been to frequency-division multiplex the sensor outputs, in the manner described by I. P. Giles, D. Uttam, B. Culshaw, and D. E. N. Davies, "Coherent Optical-Fibre Sensors With Modulated Laser Sources," Electronics Letters, Vol. 19, page 14, 1983. This approach is accomplished by frequency ramping the optical source and arranging the array geometry so that the transit time of the light from the source to a sensor and back to the central location is unique for each sensor. In this case, the array output is mixed with the source's present output, thereby producing a unique central frequency for each sensor. The environmental information is carried in the side bands about this central frequency.
One particular problem with the above-described system involves the "fly back" period when the periodic ramp signal is reset from its maximum to its minimum position. This fly back period comprises a time when system operation may not occur, since no ramp signal is present, and no meaningful results would be produced. This places some limit on the rate at which environmental conditions may change and still be reliably monitored by the sensor system.
In the case of frequency multiplexing, use of a short coherence length source typically results in a signal which includes a substantial amount of noise. In order to reduce the noise, a source having a longer coherence length may be used. However, proper operation of frequency modulation schemes normally requires modulation over a broad range of frequencies. To accomplish this, gas lasers are typically used requiring external modulators which must be fast in their operation. Use of such gas lasers and external, fast modulators increases the cost and complexity of the system, but reduces signal noise from that experienced with short coherence length sources, such as laser diodes.
Based on the above, it would be an important improvement in the art to provide a sensing system and technique for multiplexing a plurality of remote sensors without being subject to restrictions such as those identified which are inherent in the time-divison and frequency-division multiplexing schemes used in the past. Thus, the improved system should permit selection and individual monitoring of any of the sensors in the system, without the need for extensive electronics or other devices or schemes at the system output to recover signals relating to the desired sensor from among signals also relating to other sensors. A further improvement in the art would be to provide such a system which accomplishes its purpose with detection at lower frequencies such as those in the acoustic range, rather than at the higher modulation frequencies. It would be a still further improvement to provide such a system which provides minimized noise levels while requiring modulation over a narrower range and thus at a slower modulation rate than is necessary in frequency modulation using a long coherence source. Preferably, such a system should permit use of a wide range of optical sources, including continuous wave sources, and should be both simple and economical to produce and use in actual application.